Steam generators, such as the steam generators of nuclear reactors cooled by pressurized water, comprise an outer shell of cylindrical general shape placed vertically in a nuclear reactor building, that is to say with the axis of the outer shell lying vertically.
The steam generators of pressurized-water nuclear reactors heat and vaporize feedwater by heat exchange with the pressurized cooling water of the nuclear reactor that circulates inside tubes of a-heat exchange bundle. The tube bundle is placed inside a bundle wrapper of cylindrical general shape, which is placed coaxially inside the outer shell.
The tubes of the bundle are fixed at their ends in a tube plate so that they open, at a first end, into a first part of a water box of the steam generator and, at a second end, into a second part of the water box of the steam generator. The water box of the steam generator makes it possible to distribute the pressurized water coming from the nuclear reactor vessel in which the core, consisting of fuel assemblies, is placed and to recover the pressurized water that has circulated inside the exchange tubes so as to return it to the nuclear reactor vessel.
Feedwater for the steam generator is introduced through the outer shell and channelled so as to enter an inlet part of the exchange bundle, at the lower part of the bundle and of the bundle wrapper. The feedwater then flows upwards inside the bundle wrapper in contact with the external surface of the tubes in such a way that it is heated then vaporized and is in vapour form in the upper part of the outer shell of the steam generator. The steam recovered in the upper part of the steam generator passes into separator-dryers and is sent to the turbine of the reactor.
The feedwater is generally introduced into the upper part of an annular space provided between the tube bundle wrapper and the outer shell or between the bundle wrapper and a skirt defining a circulation space that communicates with an end part-of the bundle formed by the ends of the bundle tube legs called cold legs, via which the cooling water that has served for heating and vaporizing the feedwater leaves.
It is also possible to provide a feed space bounded by a skirt coaxial with the outer shell and with the bundle wrapper of the steam generator in order to preheat the feedwater by circulation in contact with the bundle wrapper and in contact with the cold legs of the bundle tubes.
In all cases, the feedwater introduced into the shell of the steam generator flows downwards in an annular space of vertical axial direction, down to the lower part of the bundle wrapper.
To achieve high efficiency of the steam generator and satisfactory operating conditions, it is necessary to distribute the flow of feedwater around the circumference of the annular feed space of the steam generator, at the upper part of this annular space. To do this, a feedwater delivery device is used at the upper axial end of the annular space, this device comprising a header of toroidal general shape approximately coaxial with the outer shell and with the annular space. The header is placed inside the outer shell of the steam generator, vertically in line with the upper part of the annular water feed space. The header is connected to a water feed pipe that passes through the outer shell of the steam generator and includes feedwater distribution means distributed around the circumference of the annular water feed space. The feedwater distribution means may be formed by a plurality of openings that pass through an upper wall of the toroidal header and are distributed circumferentially around the wall of the header, to each opening a tube of approximately vertical direction may be fixed. Each tube is fixed to the wall of the header by a lower end part, the upper end part of the tube being able to be bent so as to direct the feedwater downwards into the upper part of the annular space. In this known embodiment of the prior art, the tubes, in the form of an upside-down J, have a vertical cold leg fixed to the header and a loop directed downwards towards the upper part of the annular feed space.
The feedwater, which is fed into the header, having the shape of a torus or a portion of a torus, may contain foreign bodies consisting in particular of metal machining or welding debris or of mechanical fixing means such as bolts, particularly during the first phases of operation of the steam generator and of the secondary water feed system, after they have been manufactured in the factory and mounted on the site of the nuclear reactor. Foreign bodies may also be introduced into the annular space of the steam generator with the condensation water coming from the upper part of the steam generator containing the steam separator-dryers.
Such foreign bodies entrained by the feedwater flowing at high speed in the steam generator constitute migrating bodies that may result in the destruction of certain parts of the secondary system or of the steam generator. In particular, migrating bodies are liable to become trapped between the tubes of the steam generator bundle and damage or destroy the tubes with which they are in contact.
It has therefore been proposed to use filtration devices for stopping the foreign bodies entrained by the steam generator feedwater and liable to be entrained into the annular circulation space of the steam generator.
Such filtration devices comprise, for example, one or more screens placed in the meridional section of the header in order to stop the foreign bodies or debris circulating with the feedwater.
In the case of large debris striking the screens, such debris may be sent back towards the feed pipe of the header or may damage the screens so that the screens become less effective for filtering the feedwater.